1. Field of the Invention
The present invention is directed to a plasma process for treating a capacitor dielectric oxide, particularly the anodic oxide serving as the dielectric in an electrolytic capacitor. This treatment reduces the oxygen vacancy defects in the anodic dielectric oxide prior to incorporation of the formed (anodized) anode into the capacitor. Reduced oxygen deficiencies improve the capacitor's DC leakage as well as charge/discharge energy efficiency and long-term performance stability. These properties are important for critical applications such as implantable cardioverter defibrillators.
2. Prior Art
Electrolytic capacitors are well known for use in a variety of electronic equipment such as consumer audio and video equipment, home appliances, power supplies, industrial electronics, military electronics, computers, telecommunication equipment, entertainment equipment, automotive devices, lighting ballasts, and implantable medical devices. In general, electrolytic capacitors comprise an anode and a cathode segregated from each other by at least one layer of separator material impregnated with a working electrolyte. The anode is a valve metal body coated with a layer of the corresponding metal oxide serving as a dielectric.
The dielectric oxide in electrolytic capacitors is normally formed using a technique known as anodizing. Passing an anodic current through a valve metal immersed in an anodizing (formation) electrolyte does this. The thickness of the resulting anodic oxide is proportional to the anodizing voltage. The desired oxide thickness is determined by the capacitor rated voltage and other required properties. For a given dielectric oxide thickness, the volumetric capacitance and energy density of a capacitor are functions of the specific surface area of the valve metal anode. To increase capacitor volumetric energy density, a porous valve metal is normally used. Examples include an etched aluminum foil for an aluminum capacitor and pressed and sintered tantalum powder body for a tantalum capacitor.
The oxide quality formed by anodizing depends on a number of factors including purity of the valve metal, anode micromorphology, anode size and geometry, formation electrolyte composition, anodizing temperature, and anodizing protocols. Nonetheless, the formed oxide often contains defects due to oxygen vacancies and incorporated harmful species from the anodizing electrolytes. These adversely affect DC leakage and stability of the oxide layer.
It is undesirable for a capacitor to experience high DC leakage and oxide degradation for several reasons. For one, dielectric oxide degradation results in increased capacitor charging time, which is especially important when the capacitor is incorporated into an implantable medical device. An example is an implantable cardioverter defibrillator where the battery powers circuitry that monitors the heart. As long as the heart is beating in a normal rhythm, nothing more is needed. Heart monitoring is a relatively low energy requirement. From time to time, a tachyarrhythmia may be detected. This is an abnormally rapid heartbeat that if left uncorrected can be fatal. Upon detection of a tachyarrhythmia, the battery goes into a device activation mode where it rapidly charges the capacitor, which then dumps its load to shock the heart back into a normal beating rhythm.
In that respect, the capacitors in an implantable cardioverter defibrillator operate infrequently. While they are idle most of the time, once a tachyarrhythmia is detected, the capacitors need to be charged up quickly. But, oxide degradation results in increased capacitor charge time. Oxide degradation also decreases capacitor charge/discharge energy efficiency, which either decreases the useful life of the battery or increases battery and device volume. All these consequences are a result of defects in the anodic oxide film. The more imperfect the dielectric oxide, the longer the charge time is and the less efficient the charging and discharging.
Due to the oxide degradation, such as in an aluminum electrolytic capacitor used in an implantable cardioverter defibrillators, the capacitor must also be subjected to a so-called re-form procedure to recover the capacitor charging time by healing the degraded oxide therein. Charging the capacitor to or near its rated voltage normally does this. The charge is then emptied into a dummy circuit or allowed to slowly bleed off. Energy needed to reform the valve metal anode decreases the useful life of the battery and, consequently, the implantable defibrillator.
Heat treatment in air is one conventional method for reducing DC leakage and improving the quality of a dielectric oxide generated by an anodizing process. Heat treatment can be done after the anodizing process is completed, especially when relatively thick oxides (greater than about 100 nm in thickness) are desired, or as an intermediate step in an anodizing process. Heat treatment is also frequently used in conjunction with other coating deposition processes including reactive physical vapor deposition (RPVD), chemical vapor deposition (CVD), thermal oxidation, and oxygen plasma deposition. It is normally performed in air and at a temperature up to about 550° C. While the exact mechanism is not clear, heat treatment is believed to reduce oxygen vacancies and contamination in the oxide film, such as those caused by hydrogen, carbon and phosphorous.
Chang et al. in a publication titled “Improvement of Electrical and Reliability Properties of Tantalum oxide by High-Density Plasma (HDP) Annealing in N2O”; IEEE Electron Device Letters, vol. 23, issue. 11, pp 643-45, 2002, describe another method for treating a dielectric oxide. They propose using high-density plasma annealing in O2 and N2O on thin tantalum oxide films having a thickness of 10 nm or less deposited by chemical vapor deposition (CVD). Chang et al. write: “This study aims to improve the electrical characteristics and reliability of low-pressure chemical vapor deposited (LPCVD) tantalum pentoxide (Ta2O5) films by a new post-deposition annealing technique using high-density plasma (HDP). Experimental results indicate that excited oxygen atoms generated by N2O decomposition from HDP annealing can effectively reduce the carbon and hydrogen impurity concentrations and repair the oxygen vacancies in the as-deposited CVD Ta2O5 film, thereby resulting in a remarkable reduction of the film's leakage current. Two other post-deposition annealing conditions are compared: HDP 02 annealing and conventional plasma O2 annealing. The comparison reveals that HDP N2O annealing has the lowest leakage current and superior time-dependent dielectric breakdown (TDDB) reliability.” The problem is that oxide films deposited by chemical vapor deposition techniques as well as by physical vapor deposition (PVD) have poor stoichiometry and normally contain significant contamination. Either post deposition annealing in air or oxygen plasma treatment is needed to remove the contaminants and improve oxide stoichiometry.
Unlike films produced by CVD and PVD techniques, an oxide film, such as of tantalum oxide, formed by anodizing is believed to have nearly perfect stoichiometry and contain relatively little contamination. Defects in tantalum oxide films that cause high DC leakage and oxide degradation are believed to be mostly due to oxygen vacancies that increase in density as the film thickness increases. While anodic oxide films also contain foreign species incorporated from the electrolytes during anodizing, certain electrolyte species such as phosphates at appropriate levels are believed to be beneficial to the film's stability and capacitor long-term performance.
While these conventional heat treatment methods give satisfactory results, they are not believed to completely rid the dielectric oxide of defects, especially those in relatively thick anodic oxides. Since oxide defects adversely impact DC leakage, among other characteristics, improvements here are important. This is nowhere more critical than when the capacitor is incorporated into an implantable medical device, such as an implantable cardioverter defibrillator, where fast charging and efficient charge/discharge are paramount. The present plasma treatment method is believed to be such an improvement.